This paper deals with the stress of a cup-type flexible spline in strain wave gearing (commonly known as harmonic drive gearing) with an oval wave generator. Nominal stresses at four positions on the tooth and at three positions on the cylinder were measured using three-element rectangular rosette strain gages. Then the distribution states of circumferential stress, axial stress and shear stress on each position of the flexible spline were analyzed, giving the following results. The amplitude of the varying stress on the center of the tooth is the largest in the flexible spline. The amplitude of varying stress is also large, and the maximal and minimal stresses are nearly the same as the maximum and minimum ones respectively at the open-side end of the tooth part.